This work investigates boiling phenomena by means of imaging and characterization of bubble dynamics in the vicinity of the
bubble’s nucleation site. A silicon wafer is used as heat transfer surface so that MEMS fabrication can be applied to create
artificial cavity for prescribed nucleation site. The well-controlled bubbles growing on such nucleation site can facilitate
measurement and observation. High-speed video camera is employed in visualization, and the instantaneous thickness of the
liquid layer is recorded by a confocal optical sensor. Tests are first performed on a water layer with the thickness of
7.5mm±0.5mm, and the bubble departure diameter and frequency as well as the transient evolution of bubble diameter and foot
size are obtained in isolated bubble regime. Bubble departure diameter enlarges with increasing heat flux, and the measured
maximum diameter is around 3.2 mm. With the decrease of the liquid layer thickness to 2 mm, the bubbles are found to remain
on the heater surface for a relative long period, with a dry spot initiation under the bubble that becomes rewetted after the
bubble bursting. As the water layer thickness decreases further, irreversible dry spot appears, suggesting a minimum “safe”
film thickness in the range from 1.2 to 1.9 mm under the tested heat flux range from 26 kW/m2 to 52 kW/m2.

General Note:

The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows

This work investigates boiling phenomena by means of imaging and characterization of bubble dynamics in the vicinity of the
bubble's nucleation site. A silicon wafer is used as heat transfer surface so that MEMS fabrication can be applied to create
artificial cavity for prescribed nucleation site. The well-controlled bubbles growing on such nucleation site can facilitate
measurement and observation. High-speed video camera is employed in visualization, and the instantaneous thickness of the
liquid layer is recorded by a confocal optical sensor. Tests are first performed on a water layer with the thickness of
7.5mm+t0.5mm, and the bubble departure diameter and frequency as well as the transient evolution of bubble diameter and foot
size are obtained in isolated bubble regime. Bubble departure diameter enlarges with increasing heat flux, and the measured
maximum diameter is around 3.2 mm. With the decrease of the liquid layer thickness to 2 mm, the bubbles are found to remain
on the heater surface for a relative long period, with a dry spot initiation under the bubble that becomes rewetted after the
bubble bursting. As the water layer thickness decreases further, irreversible dry spot appears, suggesting a minimum "safe"
film thickness in the range from 1.2 to 1.9 mm under the tested heat flux range from 26 kW/m2 to 52 kW/nr.

Introduction

Boiling is a two-phase flow and heat transfer phenomenon
encountered in our everyday life (e.g., in kitchen) and many
engineering applications (e.g., utilization of thermal and
nuclear energy). During past decades, experimental studies
have been extensively performed to pursue the plwsical
mechanisms of boiling, with the objectives of understanding
and enhancement of boiling heat transfer, and prediction of
its limit (boiling crisis and CHF). It is believed that the
near-wall microlaver behaviour and bubble dynamics are of
paramount importance to boiling heat transfer.

Although high-speed photography is an emerging diagnostic
technique for study of boiling, it is still difficult to obtain
high-quality images near the heater surface due to chaotic
characteristics and random nature of bubble growth.
Artificial cavities were therefore introduced as fixed
nucleation sites to produce bubbles under relatively low
heat flux conditions so that single bubble dynamics and
bubbles coalescence can be visualized [1-5]. In most
experiments, the side views were obtained, but the top
views were obscure, due to convoluting interfaces of
departing bubbles. In attempt of characterizing macrolayer
and dry spot dynamics in the near-wall region, transparent
heaters were developed and employed to enable bottom
visualization [6-11].

In BETA experiment [12-13] micro hydrodynamics of pool
boiling was diagnosed from both side and bottom views, by
using X-ray radiography (side view) and high-resolution
infrared thermometry (bottom view). The bottom

measurement was made possible by using a thin heater
(140 nm) to obtain dynamic thermal patterns of the
heater's surface. The X-ray radiography provides the
detailed void pattern near and above the heating surface.
The BETA experiments led to suggestion of a "scale
separation" concept that high heat-flux boiling and burnout
in pool boiling are governed by micro-hydrodynamics of a
near-wall liquid layer sitting and evaporating on the heater
surface autonomously. Accordingly, the studies on boiling
mechanisms can be carried out on a thin liquid layer,
which facilitates the visualization and measurement of
boiling in detailed level. This became the rationale of
BETA-B experiment to investigate boiling phenomenon in
a thin liquid film [14] so that the micro-hydrodynamics of
the film was directly visualized by a high-speed video
camera (top view) synchronized with the IR imaging
(bottom view), without losing the key plwsics of boiling
process. Through the experimental observation, the bubble
and film dynamics was conceived as shown in Figure 1.

The present study is initiated for further qualification of
the proposed plwsical picture as illustrated in Figure 1.
High-speed, high-resolution video camera is employed in
visualization, and the instantaneous thickness of the liquid
layer is recorded by a confocal optical sensor. Artificial
cavities are created on silicon wafer by MEMS fabrication
for prescribed nucleation sites. As the first step, tests are
performed with a thick water layer (~7.5 mm thick), and
the evolution of bubble diameter and foot size are obtained
in isolated bubble regime. The focus is then placed on thin
water films (less than 2 mm) to investigate how the film
thickness and heat flux affect the water film behavior and

Figure 1: A conceptual picture of bubble collapse in a thin
liquid film on heater surface.

Nomenclature

As shown in Figure 2, the test facility consists of an optical
table, liquid supply and temperature control system, power
supply and heating system, high-speed visual system,
confocal optical sensor system, 2 one-dimensional linear
manipulators, a three-dimensional micro-manipulator and
its control system, lighting system, and a test section for
boiling on solid surfaces. The optical table provides the
required vibration isolation and serves as an ideal platform
for installation of the test section and the instrumentation.
Liquid is preheated in a stainless steel tank by a band
heater to a desired temperature and the temperature level is
maintained with a temperature controller: the hot liquid is
then supplied to the test section through a micro pump
capable of accurate flow rate control.

As shown in Figure 2, the test section is composed of a
leak tight vessel and heater surface. The vessel is made of
Teflon and has the cross-sectional area of 90 mm x 60 mm
and the depth of 15 mm. Deionized water is used in the test,
conducted at atmospheric pressure. Cartridge heaters are
immersed in the pool to control the water temperature in
the vessel. Water temperature is monitored by T-t pe
thermocouples. A glass window (30 mm x 30 mm) is
mounted in the centre of the top cover for optical sensor
thickness measurement and high speed camera
visualization. The periphery of glass window is heated by a
stainless steel film heater to avoid vapor condensation. Two
other observation windows are arranged on opposite lateral
walls. A silicon wafer as heater's surface is embedded a
centre-hollowed quartz glass sheet (2 mm thick) which is
then installed on the protuberant bottom of the vessel; see
Figures 2 and 3. Steam generated in the vessel is
discharged to a condenser and the condensed water returns
to the vessel. Such design facilitates liquid film formation
on the heater's surface and keeps the film thickness stable.

The silicon wafer with the dimensions of 30 mm x 20 mm
in area and 0.4 mm in thickness was used as the heating
surface to investigate the dynamics of bubble growing on a
single artificial cavity. The static contact angle for distilled
water dropping on the silicon wafer surface was measured
of 60 degrees. In the centre of the upward-facing surface
(see Figure 3), a cylindrical cavity with both diameter and
depth of 100 pLm was manufactured using precision
photolithography. On the downward-facing surface of the
silicon wafer, a thin layer of silicon nitride (Si3N4) with
thickness of 150 nm was coated by the LPCVD technique
for electric insulation, and then a 150 nm thick titanium

film was coated as heating element. The titanium film is
heated by DC power supply through copper electrodes
connected to the film by silver epoxy. The heating zone is
10 mm x 10 mm locating in the centre of the silicon wafer.

Under the artificial cavity, a T-type thermocouple with the
diameter of 130 pLm was fixed on the downward surface of
the silicon wafer by electrically-insulated (but thermally
well-conductive) glue. The bottom of the silicon wafer and
the vessel is well packed with insulation material.

c) Film thickness measurement system

The confocal optical sensor system employed in the present
study was made by Micro-Epsilon Company in Germany
(www.micro-epsilon.com). As illustrated in Figure 2, the
sensor is incorporated with a controller which is also
connected to a special Xenon light source. The controller
optoNCDT2431 was chosen here, which is communicated
with the computer through a software package. The sensor
IFS2431-3 selected in the present study has sampling rate
of 30 kHz, measurement range of 3 mm, spatial resolution
of 0.12 pLm, and maximum tilt angle of + 22 .

The principle and calibration of the confocal optical sensor
was documented in [15], showing a good agreement with
the measured results of a micro conductive probe, and a
promising capability to capture film dynamics.

d) Hiah-speed visualization system

A high-speed CMOS digital camera (DRS Lightning RDT
plus) with a recording speed up to 100,000 frames per
second and a tungsten spot light (DedoCool) are used for
the visualization of the bubble and liquid dynamics. For the
present tests, the recording speed of 1000 ~ 5000 frames
per second was used. The camera and the light were placed
as illustrated in Figure 4, and adjusted to have a good
contrast of the interfaces so as to see their movements.

For the single bubble growing in a thick liquid layer, the
side-view arrangement was chosen for observation through
the two opposite glass windows fixed in the lateral walls of
the test vessel. In a thin liquid layer with its thickness
comparable to bubble departure diameter, the top view was
selected to record the bubble dynamics.

Transparent glass windows

Camerasidev~iew

Figure 4: Schematic of the high-speed visualization.

e) Test matrix

Prior to experiment, the silicon wafer surface is levelled

Figure 6 shows the binary images during bubble life circles
in test runs Al and A6 (Table 1). The image at bubble
departure was used to determine the equivalent departure
diameter.

(a) Bubble life cycle on an artificial cavity in the test run
Al: Heat flux is 17 kil or, the spatial resolution is
18pm and the time interval between frames is I ms.

side-view images taken by the high-speed camera. A digital
image processing program was developed to obtain the
required parameters. As shown in Figure 5, an image of the
bubble was transferred to a binary image, based which the
volume V and equivalent diameter d,, of the bubble, and
the foot diameter dy, were calculated by Eqs. (2-4).

VI = - 13 (2)

d,, = 31 (3)

df, Y(1) (4)
where 1 is the length of unit pixel, and Y(i) is the number of
black pixels in the ith row.

horizontally by adjusting the supporting screws of the test
section until the displacement among the four corners of
the surface was less then 5 pLm measured by the confocal
optical sensor. The optical table is an ideal platform for
such operation. Distilled water is preheated and degassed
by boiling over half an hour before taking measurement.

The water is saturated under atmospheric pressure. Based
on the measured temperature T, from the thermocouple
mounted the downward surface of the silicon wafer, the
superheated temperature of the upward-facing surface can
be obtained by the following equation:

ATs = Tw- Ta 9 s (1)
s k
where Tsat is the saturated water temperature, q is the heat
flux, 6s, and k are the thickness and thermal conductivity of
the silicon wafer, respectively.

Two groups of tests were conducted in this study:

Test-1: Keep the thickness of the water layer on the
silicon wafer at 7.5 mm + 0.5 mm but gradually increase
heat flux; record the bubble dynamics by the side view of
the camera with the spatial resolution of 18 pLm and speed
of 1000 fps. The test runs are listed in Table 1.

Test-2: Boil a liquid layer of ~2 mm initial thickness and
let it to evaporate (direct steam discharge to atmosphere)
until an irreversible dry spot appeared; recording the
bubble dynamics by the top view of the camera with the
spatial resolution of 35 pLm and speed of 5000 fps. The test
runs are listed in Table 2.

Table 2: Tests in a thin water layer.

No. q (kif or) di (mm)
B2-1 26 2.194
B2-2 26 1.208

ai Image processing for an
dynamics.

Results and Discussion

The equivalent diameter and the foot diameter of a growing
bubble on the artificial cavity were analyzed from the

MA

(b) Bubble life cycle on an artificial cavity in the test run
46: Heat flux is 60 kW ur, the spatial resolution is
18pin and the time interval between fraines is 1 ins.
Figure 6: Binary images of bubbles in a life circle.

It was found that coalescence of bubbles in the vertical
direction occurs starting from the test run A3 (heat flux:
35kit nr)', i.e., the newly generated bubble will catch up
and join in the previously departed bubble. After the heat
flux elevated to a certain level (say, 69 kit\ nr)', more
nucleation sites in addition to the artificial cavity were
activated, and bubble coalescence in the horizontal
direction occurs. The following data are taken only from
the isolated bubbles produced on the artificial cavity.

Figure 7a shows the relation of bubble departure diameter
with the heat flux. In general, the bubble departure
diameter is getting bigger with increasing heat flux. It also
shows the scattering of the equivalent departure diameters
when the heat flux is higher, due to the influence of bubble
coalescence. With around 10% uncertainty, the averaged
maximum bubble departure diameter can be predicted by
Fritz [16] correlation which calculates bubble departure
diameter dd by balancing buoyancy force and surface
tension force:

d, = 0.020895 (5)

where # is the contact angle measured in degrees and 0-is
the surface tension at saturation. The static contact angle of
water formed on the heater surface at room temperature is
used here.

Figure 7b is the bubble departure frequency, which is rising
when the heat flux is increasing from the lower level. There
seems a threshold heat flux (52 kW/m ), under which a
maximum frequency reaches, and after that the frequency
reduces slightly while the bubble departure diameter
increases significantly.

Based on the data of bubble departure diameter and
frequency, the heat used to generate bubble (latent heat)
can be calculated. Figure 8 is the ratio of the latent heat to
the total heat input from the heater. Only a small portion of

Figures 9a and 9b illustrate the evolution of the bubble
equivalent diameter and the bubble foot diameter during an
entire life time of a bubble. At beginning, the bubble life
time decreases with increasing heat flux. The variation in
bubble life time is negligible after the heat flux is higher
than 35 kW/nr. The maximum value of the bubble foot
(base) diameter also increases when the heat flux is raised.
The varying rate of the bubble foot is faster for a higher

Figure 10 shows the behaviour of bubbles
water layer with the thickness of 2 mm + (
increasing heat flux. What happens is that the 1
artificial cavity sticks on the heater surface fo
long period and its rupture occurs irregul
because that the bubble diameter is comparable
layer thickness, so that the buoyancy force w
lift the bubble is greatly decreased. More
clearly shows that there is a dry spot unde:
Figure 11 shows that the process of bubble ru
spot rewetting (reversible dry spot). The accur
around the contact line is called the dam of th
dam spreads outwards when the bubble grows
forms a ring. After the bubble rupture, the da
and induces inward spreading which makes th
receding and thus the dry spot rewetting,
outward spreading which forms the ring move
case B3-1, the bubble rupture happens at 0.:
rupture hole is completely opened at 0.8 ms, a

The evolution of the dry spot is as shown in Figure 12. The
~-x-rA5 dry spot slightly expands during the bubble rupture process
-*- A6and waits for a while with a constant diameter, and then
shrinks rapidly to zero.

If the liquid film thickness decreases further, say, from
1.964 to 1.519 mm, the rewetting of the dry spot will not
take place, as shown in Figure 13, where the bubble rupture
40 50 is initiated at 0.2 ms, and the receding of the contact line
begins at 0.6 ms but stops at 2.0 ms and the dry spot is no
ng a ubble longer rewetted. This is an irreversible dry spot.

Obviously, the reversibility of the dry spot under the
der different isolated bubble regime (only single nucleate site activated
eat flux: Al: via the artificial cavity) is affected by the water layer
: 43 kW/m2, thickness and heat flux. In other words, given a specific
heat flux, there is a minimum thickness of water layer,
below which the dry spot is irreversible. Table 3 shows the
dynamics minimum thickness values, which is also plotted in Figure
14 where the corresponding maximum bubble diameter and
in a thinner dry spot diameter are also presented. The minimum liquid
0.3 mm, with layer thickness increases gradually with the rising of the
bubble on the heat flux within the tested range.
,r a relatively
arly. This is Table 3: Test results in a thin water layer.

e to the water
hich helps to
over, it also
r the bubble.
pture and dry
mulated water
e bubble. The
,and its front
m falls down
contact line
as well as
ment. For the
2 ms and the
nd finally the

An experimental study has been carried out to investigate
the dynamics of a single bubble formed on an artificial
cavity of silicon wafer covered by a water pool (~7.5 mm
deep) and a water layer (<2.3 mm thick). The confocal
optical sensor and high- speed camera are used to measure
the layer thickness and bubble dynamics.

The bubble departure diameter and frequency increases
gradually with the rising of heat flux in the water pool with
the thickness of 7.5 mm + 0.5 mm, and the maximum
departure diameter of the isolated bubble is about 3.2 mm.

When the thickness of the water layer decreases to around
2 mm, the bubble sticks on the heater surface for a relative
long period, and a dry spot turns to appear under the
bubble; later the bubble ruptures and the dry spot rewets.
Further reducing the water layer thickness, an irreversible
dry spot appears, which is related to a minimum thickness
for a given heat flux. Its value increases from 1.2 mm to
1.9 mm when heat flux increasing from 26 kW/m2 to 52
kW/m2.

For a 4 mm thick water layer at a higher heat flux (beyond
the isolated bubble regime), a large bubble is formed
hovering over a thin liquid layer where bubbles are
generated on multiple nucleation sites. The preliminary
observation of the bubble dynamics in the thin liquid film
was obtained. More effort will be focused on the dynamics
of the liquid film under the massive bubble.

Interestingly, when increasing the heat flux to 78 kW/m2 at
the initial water layer thickness of 4 mm, a large vapour
bubble was formed, hovering over a thin liquid layer which
is much thinner than the bulk water layer. In the thin liquid
film, random nucleation sites are activated and bubbles are
produced over the sites as well as the artificial cavity. When
the bulk layer decreases to 3.545 mm, an irreversible d y
spot appears after the rupture of a random bubble under the
massive bubble; see the insert at 5ms of Figure 15. During a
relative long period of time, this dry spot can not be
rewetted but stays there and expands a little, while the dry
spots under other bubbles can be rewetted. Thus, the boiling
in the thin liquid film under the large bubble provides a new
window to observe the micro interactions of near-wall
bubbles and the film. Unfortunately, the large bubble affects
the focus of the confocal optical sensor on the thin liquid
layer. The next step is to overcome the obstacle for

This study is made possible by research grant VR-2005-
5729 from Swedish Research Council (Vetenskapsradets).
The authors thank Drs. Pavel Kudinor and Liangxing Li of
KTH for their interest and many constructive suggestions,
and the staff at Nuclear Power Safety Laboratory for support
in experiment setup.